Boas are a type of
snake that are members of the Boidae family. Boas
are
basal snakes that are "primitive" in evolutionary terms
(i.e. less derived). They are constrictors and most give
birth to live young. They have anal spurs, a pair of claws
on each side of the cloaca which assist in mating. Boas are
named after cows (Latin:
bos) because of the old myth that boa snakes pursue
cows and suckle them until they are drained to death.

Boas have two subfamilies: Boinae or true boas and
Erycinae or sand boas. Pythons are sometimes classified as a
subfamily of Boidae, but are frequently listed under their
own family, Pythonidae.

Boidae

True boas are medium sized to large snakes. Females are
usually larger than their male counterparts. Boas contain
many subspecies based on locaility. They include Colombian,
Suriname, Bolivian, Peruvian, Hog Island, Long Tail
Peruvian, Argentine and more. The boas from the amazon basin
are the most colorful possessing bright cherry red tails. It
used to be said that boas were New World Snakes and pythons
were Old World Snakes, but, with boas found on Madagascar
and the Solomon Islands, this is not quite true. Instead, it
is possible that boas have survived in evolutionarily
isolated areas. South America, until a few million years
ago, had a distinct fauna that included marsupial mammals; with the land bridge to North America, boas
have migrated north as placental mammals and
colubrids (for example) have migrated south.

Erycinae

Compared to true boas, erycines are quite small, with
most members of this subfamily remaining well under a metre
in length. Fossil erycines have been found in rock strata
over 50 million years old, and were once widespread in North
America. Now, only two species remain in North America, as
well as the sand boas in Africa, Asia and southeastern
Europe.

At least three erycine species lay eggs: the Calabar
Burrowing "Python" , Calabaria reinhardtii
(once classified as a python for this reason); the Arabian
Sand Boa, Eryx jayakari; and the West African Sand
Boa, Eryx muelleri.